1. Field of the Invention
The present invention relates to a polarizer for obtaining linearly polarized light from natural light, and more particularly to a polarizer for use in an optical system such as a liquid crystal projection television, an optical communications system, or the like.
2. Description of the Prior Art
There have been known three different types of polarizers; (1) one using a double refracting crystal material, (2) one using a medium material having a linearly dichroic property, and (3) one using surface reflection on a thin film of dielectric material.
The double refracting crystal polarizers include the Nicol prism and the Glan-Thompson prism, each of which consists of two prism elements of calcite bonded together with an optical adhesive so that the ordinary ray of incident light entering the prism is totally reflected at the junction and the extraordinary ray passes through the prism as a linearly polarized light.
The linearly dichroic polarizer is composed of films of linearly dichroic material, e.g. iodine-PVA, bonded with a protective layer of e.g. acryl or triacetylcellulose. In this polarizer, one of two linearly polarized components of incoming light running straight is absorbed by the linearly dichroic material, whereby the other linearly polarized component will be obtained after passing through the dichroic material film of an adequate thickness.
The dielectric thin film polarizer is provided with a dielectric thin film interposed between two prisms for polarizing separation of light. Light entering the polarizer is separated by the dielectric thin film into two linearly polarized components; one passing through and the other being reflected. Hence, both the passing and reflected light components are delivered as linearly polarized light outputs.
However, the double refracting crystal polarizer when increased in size requires large-size crystal material, thus causing the size and production cost of a relating optical device. The linearly dichroic polarizer which absorbs a particular component of incident light permits no reflected light to be delivered for use and is disadvantageous in the resistance to heat and climate. Also, the polarizer having a dielectric thin film interposed between two prisms often has to be enlarged in the overall size in order to pass a desired amount of polarized light.
Some improved ones of the dielectric thin film polarizers which are reduced in the size or thickness are disclosed in Japanese Patent Laid-open Publications No. 61-17103(1986) and No. 61-262705(1986).
The polarizer disclosed in the JP Publication No. 61-17103 is a polarizing beam splitter provided with a relief-type diffraction grating having formed thereon a polarization dependent reflective layer. This polarizer employs a diffraction grating which allows a particular wavelength component of incident light to be diffracted and will hardly be suited for use as a general-purpose polarizer.
The polarizer disclosed in the JP Publication No. 61-262705 incorporates a multi-layer dielectric material serving as both a polarizing filter and a phase differential plate and provided on a triangle-waveform surface of a transparent material which is formed by arranging inverted-V shape crests and V-shaped recesses alternately. In other words, the triangle-waveform surface of a transparent resin layer provided on a substrate is coated with a multi-layer film of dielectric material. However, the forming of the multi-layer dielectric film on the resin layer is difficult, and the use of a resin material will lower the resistance to climate. Also, both the peak and bottom points of the triangle-waveform cause reduction in the performance of the polarizer. Further, the wavelength selection capability is insufficient.
As described above, the conventional polarizers are disadvantageous in the production cost of a relating device, the use of reflected light, the reduction of overall size, and the resistance to climate and also, are insufficient in such optical properties as the extinction ratio and the wavelength selection.